In the manufacture of reinforced rubber products, such as automobile tires, it is important to have good adhesion between the rubber and the reinforcing material. Originally, adhesion of the rubber to the reinforcing material was promoted by pretreating the reinforcing material with certain adhesives. This proved unsatisfactory and it is now conventional to incorporate into the rubber during compounding various chemicals that react to improve the adhesion of the reinforcing materials and rubber. This compounding adhesion method is now generally practiced regardless of whether the reinforcing materials are pretreated with adhesives.
The conventional method of compounding adhesion comprises compounding into the rubber before vulcanization a two part adhesive system. One part is a methylene donor compound that generates formaldehyde upon heating. The other part of the adhesive system is a methylene acceptor compound. During the vulcanization step the methylene donor upon heating releases formaldehyde and the methylene acceptor reacts with the formaldehyde, rubber and reinforcing material with a resultant increase in adhesion of the rubber to the reinforcing materials. In addition, proper selection of the methylene donor and methylene acceptor can improve many other properties of the final product. The methylene donor and the methylene acceptor are compounded into the rubber and thus have a significant effect on the process of making the reinforced rubber product.
Many different methylene acceptor compounds have been tried with various degrees of commercial success. Examples of the most common methylene acceptor compounds are resorcinol, resorcinol formaldehyde novolak resins, phenol formaldehyde novolak resins and phenol resorcinol formaldehyde novolak resins. Pure resorcinol is used in combination with methylene donors, such as hexamethylenetetramine (“HEXA”) or hexamethoxymethylmelamine (HMMM). The resins condensed in this manner increase the strengthening effect and improve the interfacial adhesion while at the same time providing increased rigidity of the rubber blend and improved wear resistance of the tread under a wide range of stress conditions. However, the use of resorcinol alone presents health and environmental protection problems because resorcinol may fume under the rubber processing conditions. In contrast to free resorcinol, resorcinol formaldehyde novolak resins release substantially less vapors and are used in combination with HMMM, although they are less effective for the adhesion than free resorcinol. At the same time, the stress values and especially the Shore A hardness of the strengthened rubber blends modified with such resorcinol novolaks are substantially less than those achieved when free resorcinol is used. Furthermore, owing to the strong natural color, resorcinol-formaldehyde resins cannot be used in those rubber blends intended for applications in which a pale color of the vulcanized rubber is required. Another concern is the processability of the rubber compound. Adding a methylene acceptor, such as a resorcinol-formaldehyde resin, and a methylene donor, such as HMMM, to a rubber compound tend to increase the viscosity of the uncured rubber compound, and thus the processability is decreased.
Therefore, there is a need for resorcinol resins that do not produce volatiles such as resorcinol at Banbury mixing temperatures, are not hygroscopic and do not bloom. Furthermore, there is a need for resorcinol resins for rubber compounding which yield good processability without sacrificing other desired performance properties.